Just got a gray one in the mail and its actually nice. Added grip, can generally still see the back of the M8 and protects from light nicks/scratches around the back, up the sides, pretty much up to the screen. Its advertised as .3mm thick which I believe. I've had mine on for a couple days and I already got a pretty healthy scratch in it, but I just ordered three more for like $10 shipped, so its whatever.
At any rate, just thought I'd share.